The present disclosure relates to an electrostatic actuators/transducers and micro-electro-mechanical devices that have an electrostatic actuator, particularly to micromachined ultrasonic transducers (MUT) such as capacitance micromachined ultrasonic transducers (cMUT).
Electrostatic actuators/transducers are widely used in various micro-electro-mechanical system (MEMS) devices. An electrostatic actuator/transducer generally has at least two electrodes. An electrical field is applied between the two electrodes to move one or both of the electrodes or to detect the movement of the electrodes. In general, electrostatic actuators/transducers can be classified into two types: parallel plate actuators/transducers and electrostatic comb drivers.
A parallel electrostatic actuator/transducer is usually made of two electrodes which have two surfaces faces each other. At least one of two electrodes moves along the direction of applied electrical field. The electrode may move slightly off the direction of the applied electrical field if the electrode has non-uniform displacement profile. The parallel actuator/transducer is simple and occupies little space. It can usually also generate relative large electrostatic force or force density.
Parallel electrostatic actuators/transducers have been adapted to micromachined ultrasonic transducers (MUT) and used being ultrasound applications. Various parallel-types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. Other MUTs using piezoelectric (pMUT) and magnetic (mMUT) transducers are also adopted.
However, parallel actuators/transducers usually have very limited displacement range. The direction of the transducer displacement is along the direction of electrical field, and the maximum displacement in this direction is limited by electrode gap g defined as the shortest distance between two electrodes even in an idea parallel plate system with a constant spring loading. When a voltage V is applied between the two electrodes, the electrostatic force ƒ generated in a unit area is ƒ=∈V2/(2g2), where ∈ is the dielectric constant. The electrostatic force generated by a parallel electrostatic actuator/transducer is therefore very nonlinear as function of the electrode gap g. Moreover, a parallel plate actuator/transducer usually has a collapse voltage which further limits the displacement to only a portion, e.g. one third, of the electrode gap g.
Electrostatic comb drivers are known to have a potential to overcome the nonlinear and displacement limitations of parallel actuators/transducers. An electrostatic actuator/transducer based on a comb driver generates an electrostatic force to laterally (vertically) move a movable member of the comb driver. But the existing electrostatic comb drivers have their own challenges. For example, a comb driver usually occupies more space than that of a parallel plate actuator and has smaller electrostatic force or force density. Ideally, the width of the comb fingers should be as small as possible to enhance the force density of the comb driver. But because a certain combination of conductivity and mechanical strength is needed to maintain proper function of the comb driver, a trade-off is usually done for the design of the comb finger width.
Furthermore, electrostatic comb drivers have not been used in micromachined ultrasonic transducers, especially ultrasonic applications such as cMUTs.